I have a Windows XP system. I have installed the latest version of Quicktime (7.7.6) and the latest version of Firefox, but the plugin in will NOT play the video portion (audio OK) for some on-line .mov files. For example, on my own website, www.space-machines.com, this video plays fine: http://www.space-machines.com/movies/Ariel%20III.mov but this one does not: http://www.space-machines.com/movies/Ariel%20I.mov I am not able to detect any pattern in which videos will play correctly and which not, except that the older the video, the less likely it is to work. (You can scroll through my web site to test this for yourself -- videos are listed in order of most recent first.) BTW, this is a problem which has cropped up just recently, which is the reason I was motivated to download and install the latest versions of QT and FF -- but it has not helped. And the really frustrating thing is that all of this has been working perfectly for years!!! There is, however, one suspicious coincidence: even though Microsoft is said to have discontinued support for XP, myself and other XP users still get occasional requests from Microsoft to download updates or fixes when we shut down our computers at night, and I always honor such requests. Well, I got such a request just last week (on May 15th or thereabouts), and so as usual I clicked "Yes, OK to update before shutdown." I have since reverted to an earlier checkpointed version of my Windows XP, but that has not fixed the problem.